1. Field of the Invention
This invention relates to biomimetic composites having integrated organic and inorganic phases, methods of synthesis for producing the biomimetic composites, and use in biomimetic nanostructured materials for repair and regeneration of mineralized tissue.
2. Background Information
Two examples of mineralized tissue include bone and dentin. Both of these are highly organized hierarchical nanocomposites in which mineral and organic phases interface at a molecular level. There are materials known in the art for use in repairing mineralized tissue. However, there are disadvantages associated with these known materials. For instance, there are known graft materials consisting of ceramic powders or physical blends of mineral and organic phases. These materials have been found to exhibit mechanical properties significantly inferior to those of actual mineralized tissue.
Mineralized tissues, such as bone and dentin are unique, hierarchical nanocomposites which can include about 70% by weight carbonated apatite, 20-25% by weight organic matrix, and 5-10% by weight water. Mineralized collagen fibrils are the major organic component of these tissues. Other non-collagenous proteins (NCPs) and glycoproteins account for less than about 10% of the total organic content and contribute to the regulation of mineralization, cell signaling and mechanical performance of the tissue.
Thus, the basic building blocks of bone and dentin are mineralized collagen fibrils, which are the first level of structural hierarchy of these tissues. Mineralized collagen fibrils contain stacks of plate-shaped crystallites of carbonated apatite. These crystallites can be about 3-5 nm thick, about 50 to 100 nm in two other dimensions, and aligned with their crystallographic c-axes along a fibril axis. It has been shown that the mineral component in these fibrils has almost two times greater strain than geologic or synthetic apatite and the organic component is significantly stiffer than nonmineralized collagen. These differences may be due in part to one or more of the following: (i) the nanoscopic dimensions of the crystallites, (ii) the plate-like shape of the crystallites which leads to insensitivity of these nanocrystals to flaws, and (iii) extremely high surface-to-bulk ratio which translates into high strain values.
Furthermore, the interlaced structure of the mineralized collagen fibrils creates intimate interactions of the mineral crystallites with collagen triple helices resulting in an unique mineral-organic interface at the molecular level.
The mineralized tissues have a complex organization and unique mechanical properties. In contrast, known composite bone-grafting materials are simple physical blends of organic and mineral phases. Therefore, it is highly desirable to design novel nanomaterials in accordance with the structure and properties of the mineralized tissues.
NCPs are involved in collagen mineralization. A characteristic of NCPs is the disproportionately large percentage of acidic amino acids such as Asp, Glu and Ser(P). For example, the major NCP in dentin is phosphophoryn (DPP). DPP includes primarily Ser-Ser-Asp repeat motifs with more than 90% of serines phosphorylated. Protein phosphorylation is one of the most common post-translational modifications. However, the vast majority of phosphorylated proteins contain only a small amount of phosphorylation sites adjacent to kinase-specific recognition motifs. The precise phosphorylation mechanisms of the highly phosphorylated proteins from the mineralized tissues are not adequately understood in the art. It has been proposed that casein kinases (CK) 1 and 2 phosphorylate DPP intracellularly in the endoplasmic reticulum. According to certain hypotheses, phosphorylation occurs via a chain or hierarchical reaction wherein one phosphorylated serine becomes a part of the CK recognition site which leads to subsequent phosphorylation of new serines. CK transfer γ-phosphate of ATP (or GTP) to the hydroxyl group of serine or threonine, or to the phenolic hydroxyl on tyrosine residues in proteins.
A number of peptides mimicking NCPs have been synthesized. The synthesis included introducing phosphorylated amino acids during the synthesis phase. However, this approach for synthesis of bioinspired peptides has limitations. For example, introducing any single phosphorylated amino-acids during peptide synthesis leads to a significant decrease in yield, thereby limiting the total number of phosphorylated amino acids that can be added to a peptide.